1. Field
This invention relates generally to semiconductor device packages, and more specifically to leads of flat-pack no-lead semiconductor device packages.
2. Related Art
A semiconductor device may be mounted on a lead frame and encapsulated in a semiconductor device package (hereinafter “package”). A package utilizes leads for externally providing and receiving signals and power. One type of package is a flat-pack no-lead package where the leads are exposed at a bottom and at a side of the package.
A lead frame strip (hereinafter “strip”) is populated with multiple lead frames. An encapsulating mold compound and the leads in a strip are cut during singulation of the strip to create individual packages.
One type of wettable flank of a lead of a flat-pack no-lead package includes a cavity, or recess, on the end of the lead, which has been plated, such as with matte tin, nickel palladium gold or palladium, so that solder can wet to the recess. Flat-pack no-lead packages with wettable flanks have better solder fillet formation and allow for easier visual inspection of a solder joint after surface mounting of the package on a printed circuit board (hereinafter “PCB”). After singulation, a recess of a wettable flank appears as a volume of metal missing from a central area of a bottom, external corner of a lead. A recess is narrower than a lead so as to prevent the mold compound from filling the recess. The wettable flank produces a wettable surface that is higher than a bottom of the package. The wettable flank facilitates formation of a fillet. Surface tension causes the solder to wet up into the recess of the wettable flank, and the solder may advantageously form a fillet. A fillet is an extension of the solder joint at a side of a package that can be visually inspected.
Typically, two recesses are formed from a depression in a strip. The process of forming a depression is part of a process of forming the lead frames of a strip and is typically done by a lead frame manufacturer. The depression can be created by a partial-etch, or half-etch, process during manufacture of the lead frames. One known depression is shaped as an elongated slot on a bottom surface of a lead of one lead frame and on a bottom surface of a lead of an adjacent lead frame and on a bottom surface of an intermediate portion of the strip between the adjacent lead frames.
Saw singulation cuts through portions of the strip between lead frames. The cutting process removes, as swarf, much of the intermediate portion of the leads including a middle portion of the depression. The remaining end portions of the depression become the recesses of the wettable flanks after singulation. Most leads are copper. During saw singulation, copper may, due to the ductile nature of copper, disadvantageously fill a portion of the depression that becomes (after singulation) the recess of a wettable flank. The copper may peel when a blade of a saw arrives at an edge of a depression and may adhere to the edge of the depression. As a result, the copper debris may reduce at least one of the dimensions of the recess. Such reduction in the at least one dimension of the recess is most apparent when the recess is small to begin with. The metal of a lead frame is unsupported at the depression, and, as a result, when the strip is saw singulated, burrs or tear-outs form, and the depression also captures saw debris, such as epoxy from the mold compound. Such copper and saw debris in the recesses of wettable flanks can result in a visual rejection of a package as well as an increased risk of defects in solder joints formed during surface mounting of the package to a PCB. The debris may detrimentally affect solder joint formation. Additionally, the debris may fall out of a recess on to the PCB.
One known method to avoid debris in recesses of wettable flanks is to punch singulate, rather than to saw singulate, flat-pack no-lead packages when the pitch is less than 1 mm because of the copper debris that forms in the recess during saw singulation. However, punch singulation is disadvantageous because a number of individual units on a strip when a strip is to be punch singulated cannot be a large as a number of individual units on a strip when the strip is to be saw singulated.
Another known method reduces a rate of sawing and/or uses specialized blades in an attempt to reduce debris in recesses; however, such known methods do not eliminate accumulation and retention of debris in recesses.
Another known method uses a structure that includes a through hole opening in a lead, and then fills the through hole opening with solder prior to singulation.